try restricting cryptography only on Py<3.7

re: pyca/cryptography#5771

Author: alexcjohnson

requires-testing.txt

@@ -6,6 +6,7 @@ pytest-mock>=2.0.0,<3;python_version=="2.7"
 lxml>=4.6.2
 selenium>=3.141.0
 percy>=2.0.2
+cryptography<3.4;python_version<"3.7"
 requests[security]>=2.21.0
 beautifulsoup4>=4.8.2,<=4.9.3;python_version=="2.7"
 beautifulsoup4>=4.8.2;python_version>="3.0"